Magnetic actuators for selection devices in hosiery knitting machines or the like are known.
Generally, these magnetic actuators are composed substantially of a main magnet, which has two poles arranged side by side and separated by a discontinuity, and a selection electromagnet, which is provided with a ferromagnetic core with at least one pole located at the discontinuity between the two poles of the main magnet. The selection electromagnet is equipped with a coil, which can be supplied with electric power in order to eliminate or reduce substantially the magnetic attraction force of the pole of the core of the selection electromagnet induced by the main magnet.
Selection devices which use these magnetic actuators generally comprise a plurality of selection elements, made of a material which can be attracted magnetically, which can move with respect to the magnetic actuator along an actuation direction and which, in their motion, face the poles of the main magnet and of the core of the selection electromagnet.
The magnetic actuator is arranged on the machine so that the poles are arranged sequentially along the direction of actuation of the selection elements so that said elements, in their motion along the actuation direction, face in succession, with one of their sides, first one of the poles of the main magnet and then the discontinuity and the pole of the core of the selection electromagnet and finally the other pole of the main magnet.
Moreover, the selection elements can move from a first position, in which they are adjacent to, or even in contact with, said poles, to a second position, in which they are spaced from the poles with respect to the first position. This mobility of the selection elements in the two positions corresponds to two different actuations of the elements of the machine, generally needles, which must be selected by means of the selection device.
In practice, in many cases the movement of the selection elements from the second position to the first position is contrasted by an elastic element, which tends to keep the corresponding selection element in the second position. Upstream of the magnetic actuator, an abutment, constituted generally by a cam, acts on the selection elements so that they all reach the first position directly ahead of, or at, the first pole of the main magnet which retains the selection elements in this position until the discontinuity begins. At the discontinuity, if the coil is supplied with electric power, the attraction force of the core of the selection electromagnet generated by the main magnet is eliminated or substantially reduced to such an extent as to be insufficient to contrast the force of the elastic element which causes the transfer of the selection element into the second position, in which it remains also during transit at the second pole of the main magnet, the attraction force of which is in itself insufficient to cause the transfer of the selection element from the second position to the first position. If, vice versa, the coil of the selection electromagnet is not supplied with power, the core of the selection electromagnet retains in the first position the selection element, which remains in this position and is kept in said first position also during transit at the second pole of the main magnet.
The selection element, depending on whether it is in the first position or in the second position, consequently engaging or not other elements of the machine, causes a different actuation of the element, generally a needle, of the machine which is correlated thereto, achieving the required selection.
In magnetic actuators of this kind, difficulties are observed in sizing and feeding the coil of the selection electromagnet, since in order to achieve a precise effect on the selection elements the intensity of the magnetic field induced by the electric power supply of the coil must be, at the discontinuity, i.e., at the pole of the core of electromagnet, substantially equal and opposite with respect to that of the permanent magnetic field induced in the core of the selection electromagnet by the main magnet. If the intensity of the magnetic field induced by the power supply of the coil is significantly lower than, or higher than, the intensity of the permanent magnetic field induced in the core of the selection electromagnet, the pole of the core of the selection electromagnet still applies an attraction of the selection element, obtaining an effect which is the opposite of the intended one.
The sizing and power supply of the coil of the selection electromagnet in known types of magnetic actuators are complicated, since the intensity of the magnetic field induced at the pole of the selection electromagnet, and therefore the attraction force applied by this pole to the selection element, varies according to the number of selection elements which are in contact with, or adjacent to, the poles of the main magnet, since the selection elements produce, due to their presence, a variation of the magnetic field of the main magnet, which in turn causes variations of the magnetic field induced in the core of the selection electromagnet located inside the main magnet, at the discontinuity, between its poles.
In known types of magnetic actuators, in order to avoid selection errors, it would be necessary to supply the coil of the selection electromagnet with a current whose intensity can vary according to the various selection conditions, with considerable increases as regards the management of the actuation of the magnetic actuators.
Moreover, magnetic actuators, in the specific application to selection devices, must not hinder the positioning of other elements required for the operation of the machine. For this reason, the design of these magnetic actuators has always been oriented toward containing the overall space occupation of the magnetic actuator. This goal, in known types of magnetic actuators, is achieved by using small main magnets, which accordingly have a low power, with the consequence of having very small gaps which are comparable with the processing and assembly tolerances.
This fact forces high precision in production and assembly of magnetic actuators, which increases the corresponding costs and makes it difficult to obtain a constant behavior among magnetic actuators of equal power.
On the other hand, in order to have larger gaps and therefore solve this problem, one might consider increasing the power of the main magnet, but this, due to the way in which known types of magnetic actuators are designed, would force an increase in the dimensions of the coil of the selection electromagnet, generating other problems in terms of space occupation of the magnetic actuator.